In order to provide a pipeline pig whose location in a pipeline can be determined from outside the pipeline, it is already known to provide a magnetic attachment comprising a collar of intensely magnetised steel blocks with steel pole plates arranged to saturate the entire pipe wall and give rise to a detachable leakage field. In general, the pole plates are so arranged that the direction of the field in the pipe wall is parallel to the axis of the pipeline.
This arrangement suffers from the disadvantage that, in order to ensure adequate magnetic coupling between the pole structure and the pipe wall, the diameter of the magnetic collar must be not less than about 90% of the internal diameter of the pipe. This means that the pig cannot pass obstructions which reduce the diameter of the pipe by more than 10%. Consequently, its use is restricted to applications where a gauge plate is used or for batch separating and swabbing duties. It is not feasible to use the pig to make a record of obstructions and distortions in the pipeline without stopping at any particular obstruction or distortion. In addition, the magnetic collar is very heavy. This means that the seals, which enable the pig to be propelled by pumping liquid from behind it, are subject to excessive wear and that the pig tends to travel eccentrically with respect to the pipeline so that it is more sensitive to obstructions in the bottom of the pipeline than in the top.